Liquid crystal display devices employ the optical anisotropy and dielectric anisotropy of liquid crystal compounds. As their display mode, twisted nematic mode (TN mode), super twisted nematic mode (STN mode), dynamic scattering mode (DS mode), guest-host mode (GH mode), and DAP mode are known. As their driving mode, static driving mode, time sharing addressing mode, active matrix driving mode, and dual frequency driving mode are known.
Whereas the properties of liquid crystalline compounds used for these liquid crystal display devices are different depending on their uses, it is required of any of the liquid crystalline compounds that they are stable against external environmental factors such as moisture, air, heat, and light, and that they exhibit a liquid crystal phase at a range of temperatures as wide as possible with room temperature preferably being at its center.
Liquid crystal compositions are composed of several or twenty-odd liquid crystalline compounds to develop most suitable characteristics required of a particular display device. Therefore, liquid crystalline compounds are required to be excellent in miscibility with other liquid crystalline compounds, even in the miscibility at low temperatures particularly from the latest demand for their use in various environment.
Especially, steep threshold characteristics are required of liquid crystal compositions used in STN driving to actualize high picture quality. The steepness is a function of the ratio of elastic constants K.sub.33 /K.sub.11, and it is known that the larger the ratio of the elastic constants of liquid crystalline compounds are used in a liquid crystal composition, the steeper threshold characteristics the composition exhibits (F. Leenhouts et al., Proceedings of the Japan Display, 388 (1986) ).
Also, it is necessary to use a liquid crystal composition having a high response speed for realizing a large screen of display device. It is known that the response speed is a function of viscosity (E. Jakeman et al., Phys. Lett., 39A, 69 (1972) ). That is, it is important to use liquid crystalline compounds having a low viscosity necessary for actualizing a composition having a low viscosity.
As the compound having a large elastic constant ratio K.sub.33 /K.sub.11, compounds which have an alkenyl moiety are known. That is, they are the compounds described in M. Schadt et al., Mol. Cryst. Liq. Cryst., 122 (1985) and Laid-open Japanese Patent Publication No. Sho 61-83136; and the compounds having introduced fluorine atom and described in Japanese Patent Application No. Hei 6-92740, each of which are shown below.
______________________________________ ##STR2## K33/K11 2.28 ##STR3## K33/K11 1.78 ##STR4## K33/K11 2.28 ##STR5## K33/K11 1.90 ______________________________________
Any of these alkenyl compounds has a large ratio of elastic constants K.sub.33 /K.sub.11 (about 1.78 to 2.28) and liquid crystal compositions comprising these compounds exhibit a preferable steepness. (Elastic constant ratio K.sub.33 /K.sub.11 of the four compounds mentioned above are the values determined under the same conditions as in Example 7 (Use Example 1) mentioned below.) However, as the demand on the displaying ability of liquid crystal display devices has increased, liquid crystal compositions having a higher response speed, in other words, liquid crystal compositions having a lower viscosity have been demanded.
As the compounds having a large elastic constant ratio K.sub.33 /K.sub.11, compounds having 1,3-butadienyl group are described in Laid Open Japanese Patent Publication No. Hei 6-151445. ##STR6##
However, since the substituent, 1,3-butadienyl group has a conjugated diene moiety, the compound is chemically very unstable, and thus it was impossible to use the compound in liquid crystal compositions to be practically used.
In short, liquid crystalline compounds are long-awaited which have a large elastic constant ratio K.sub.33 /K.sub.11, still lower viscosity compared with that of known liquid crystalline compounds, high chemical stability, and excellent miscibility with other liquid crystalline compounds.